Songbird Relationships to Shrub-Steppe Ecological Site Characteristics Mary I

Rangeland Ecol Manage 64:109–118 | March 2011 | DOI: 10.2111/REM-D-10-00076.1

Songbird Relationships to Shrub-Steppe Ecological Site Characteristics Mary I. Williams,1 Ginger B. Paige,2 Thomas L. Thurow,3 Ann L. Hild,3 and Kenneth G. Gerow4

Authors are 1Postdoctoral Research Associate, University of Wyoming, Laramie, WY 82071, USA; 2Associate Professor and 3Professors, Department of Renewable Resources, University of Wyoming, Laramie, WY 82071, USA; and 4Professor, Department of Statistics, University of Wyoming, Laramie, WY 82071, USA.

Abstract Rangeland managers are often faced with the complex challenge of managing sites for multiple uses and for the diverse interests of stakeholders. Standardized monitoring methods that can be used and understood by different agencies and stakeholders would aid management for long-term sustainability of rangelands. In the United States, federal land management agencies have recently based their assessments of rangeland health and integrity on state-and-transition models to consider management trajectories. Ecological sites provide a foundation for these efforts but have not been used to address wildlife habitat. Habitat preferences are documented for North American shrub-steppe songbirds but have yet to be related to ecological sites and site characteristics. We characterized ecological sites at Browns Park National Wildlife Refuge, Colorado, using established rangeland monitoring methods to test whether 1) songbird species density and diversity differ among adjacent shrub-steppe ecological sites and 2) quantifiable ecological site characteristics could be identified that account for significant variation in songbird density and diversity. Vegetation structure (represented as basal and canopy gaps, cover, height, and shrub density) differentiated the four ecological sites and was related to songbird density and diversity. Sage sparrows (Amphispiza belli) and vesper sparrows (Pooecetes gramineu) selected habitat based on horizontal characteristics of vegetation structure, such as basal and canopy gap and plant species cover. Brewer’s sparrow (Spizella breweri), lark sparrow (Chondestes grammacus), and songbird diversity were more strongly related to vegetation structure of the plant communities than to plant composition. Our results support use of ecological sites as management units to characterize songbird habitat given that songbird density and diversity were related to site vegetation characteristics. By incorporating basal and canopy gap, height, plant cover, and shrub density monitoring methods into ecological site descriptions, managers would be provided with additional tools to assist in differentiating songbird habitat.

Resumen El manejo de pastizales naturales frecuentemente presenta el desafıó complejo de utilizar sitios para mu´ltiples finalidades y atender a una diversidad de intereses de usuarios. Me´todos de monitoreo estandarizados que puedan ser utilizados y comprendidos por diferentes agencias del gobierno y por usuarios interesados podrıán ayudar a implementar praćticas de manejo que logren la sustentabilidad de largo plazo de los pastizales. En los Estados Unidos, las agencias federales que manejan tierras a´ridas, han comenzado a basar sus evaluaciones sobre el estado de salud y la integridad de los pastizales en modelos de estados-y-transiciones para considerar trayectorias de manejo. Los sitios ecolo´gicos proveen el cimiento para dichos esfuerzos, pero no han sido utilizados en lo referente al ha´bitat de la fauna silvestre. Las preferencias de ha´bitat de aves cantoras de las estepas arbustivas de Ame´rica del Norte han sido documentadas pero no han sido auń descriptas en el marco de los sitios ecolo´gicos y sus caracterı´sticas. Se caracterizaron los sitios ecolo´gicos del Refugio Faunı´stico de Browns Park en Colorado utilizando me´todos establecidos de monitoreo de pastizales naturales para determinar si 1) existen diferencias en la densidad y diversidad de especies de aves cantoras entre sitios de estepa arbustiva adyacentes y 2) existen diferencias cuantificables en las caracterı´sticas de los sitios ecolo´gicos que expliquen variaciones significativas en la densidad y diversidad de aves cantoras. La estructura de la vegetacioń (representada en te´rmino de aberturas basales y de canopeo, cobertura, altura, y densidad de arbustos) diferencio´ a los cuatro sitios ecolo´gicos y estuvo asociada a la diversidad y densidad de aves cantoras. Dos especies de aves (Amphispiza belli y Pooecetes gramineu) seleccionaron ha´bitat sobre la base de las caracterı´sticas horizontales de estructura de la vegetacioń tales como las aberturas basales y de canopeo y la cobertura de especies vegetales. Otras dos especies (Spizella breweri y Chondestes grammacus) y la diversidad de aves cantoras estuvieron ma´s fuertemente asociadas a la estructura de la vegetacioń que a la composicioń de botańica de la misma. Nuestros resultados respaldan el uso de sitios ecolo´gicos como unidades de manejo para caracterizar el ha´bitat de aves cantoras dado que la densidad y diversidad de las mismas estuvo asociada a las caracterı´sticas de la vegetacioń del sitio. Al incorporar me´todos de monitoreo de aberturas basales y de canopeo, altura de la vegetacioń, cobertura vegetal, y densidad de arbustos en la descripcioń de sitios ecolo´gicos, quienes manejan los pastizales dispondrıán de herramientas adicionales para ayudar a diferenciar ha´bitats de aves cantoras. Key Words: gap intercept, songbird diversity, species composition, vegetation structure, wildlife habitat, Wyoming big sagebrush

Research was funded in part by the US Fish and Wildlife Service, US Geological Survey, and University of Wyoming. Correspondence: Mary Williams, Dept of Renewable Resources, 3354, University of Wyoming, Laramie, WY 82071, USA. Email: [email protected] Manuscript received 27 April 2010; manuscript accepted 6 December 2010.

RANGELAND ECOLOGY & MANAGEMENT 64(2) March 2011 109 INTRODUCTION characteristics can be identified that account for significant variation in songbird density and diversity. Clear descriptions of ecological site characteristics are essential for inventory and management of rangeland natural resources. In response to the call for revised methods to inventory and monitor US public rangeland almost 20 yr ago (National MATERIALS AND METHODS Research Council 1994), many US land management agencies (e.g., Natural Resources Conservation Service [NRCS], Bureau Study Area of Land Management [BLM], and Forest Service) have united The study was conducted on Browns Park National Wildlife their efforts to standardize assessments of rangeland health and Refuge, established in 1963 by the US Fish and Wildlife integrity and to consider management trajectories (Pellant et al. Service (USFWS), and adjacent public lands in Moffat County, 2005). Together, managers and researchers (e.g., US Geological Colorado (lat 40u489N, long 108u559E). The study area, Survey and Agricultural Research Service) have developed a approximately 8 120 ha (60% USFWS, 32% BLM, and 8% systematic rangeland assessment that incorporates state-and- Colorado State Land Board) has an average elevation of transition concepts of ecosystem function and is based on the 1 633 m. Primary land uses of the study area are wildlife identification and description of ecological sites (Pellant et al. habitat, cattle production, and recreation. Average annual 2005). Ecological sites are land areas with specific soil, temperature is 7.4uC with average annual winter, spring, and topography, and climate that differ from each other in their summer temperatures of 23.7uC, 7.2uC, and 18.6uC, respec- ability to produce distinctive kinds and amounts of vegetation tively. Average annual precipitation is 216 mm, most of which and in their response to management actions (Pellant et al. comes in the spring and fall. Elevation, soil, and vegetation 2005). Ecological site descriptions are developed using site data of the refuge and surrounding areas were integrated into characteristics and observations of local experts. Once de- a geographic information system to identify ecological sites. scribed, state-and-transition models are developed for each We obtained spatial soil data from SSURGO for Moffat ecological site to model common and potential plant commu- County (Soil Survey Staff 2006) and vegetation data (30-m nities that may occur in response to ecosystem drivers (e.g., resolution) from the National Wildlife Refuge System. The climate and management; Bestelmeyer et al. 2003). Within this study area is in the 180–230-mm precipitation zone of NRCS standardized methodology, ecological sites are the basic land Major Land Resource Area 34A-Central Desertic Basins, classification units for documenting soil, hydrological, and Mountains and Plateaus, which extends into Wyoming and biological characteristics of current and potential condition Utah. We concentrated our study on four shrub-steppe (Peacock and Caudle 2005). Ecological site descriptions are ecological sites: Loamy, Saline Lowland, Sandy, and Sandy- being developed across the western United States and provide a Skeletal (Fig. 1). Soil pits from each ecological site were used communication and planning tool for land managers to assess to determine surface and subsurface textures, depth, and site function. Wildlife habitat characteristics have not yet been salinity. explicitly incorporated into ecological site descriptions but would enhance this critical component of management of planning efforts. Site Characteristic Monitoring In the North American shrub-steppe, changes in vegetation We measured site characteristics in 107 100-m-radius plots structure and function associated with exotic invasions, altered (3.14 ha) stratified by ecological sites at $ 250 m apart. Each fire frequency, and mechanical interventions can be linked to plot was sampled in 2006 and 2007 between May and August. the decline of songbird populations such as Brewer’s sparrow Monitoring methods were adapted from Herrick et al. (2005a, (Spizella breweri) and sage sparrow (Amphispiza belli; Paige 2005b) to capture the horizontal and vertical structure of site and Ritter 1999; Knick et al. 2003). Although habitat characteristics along three 50-m line transects arranged in a preference has been documented for some shrub-steppe spoke design in each plot. Plant taxonomic names follow songbirds, such preferences have not been recorded within an PLANTS database (US Department of Agriculture, NRCS ecological site context. Individual songbird species have unique 2010). Line-point intercept was used to measure type and habitat affinities, and in a few cases, shrub-steppe songbirds amount of cover (dead and live vegetation by species, litter, can be closely tied to soil type (Dobler et al. 1996; Vander biological crust, bare ground, and rock) and top canopy height Haegen et al. 2000), suggesting that ecological sites, which are intercepted at 50 points (100-cm spacing) along each transect. based on soils, are appropriate units for assessing songbird Gap intercept measured spaces (. 20 cm) between bases and habitat conditions. Given the national effort to base monitoring canopies of two groups: all plants (annuals, perennials, and standards on ecological sites, we conducted a study to link shrubs) and shrubs. Shrub gaps were distinguished from all songbird habitat use to quantitative characteristics within and plants to characterize horizontal shrub structure. To capture among ecological sites. Inclusion of wildlife habitat character- vertical shrub structure, height of intersecting shrub canopies istics in ecological site descriptions would be a powerful step in was measured to the nearest centimeter. Basal and canopy gaps facilitating monitoring and management. were calculated separately as the percentage of line covered in We quantified characteristics on four shrub-steppe ecological gaps 20–50, 51–100, 101–200, and . 200 cm in length. Shrub sites in northwestern Colorado using established rangeland density (shrubs ? ha21) was measured in a 2 3 50 m belt on each monitoring methods to test the hypotheses that 1) songbird line transect in the plot. Shrubs within the belt transect were species density and diversity differ among adjacent shrub- grouped by species into four height classes (, 10, 10–50, 51– steppe ecological sites and 2) quantifiable ecological site 100, and . 100 cm).

110 Rangeland Ecology & Management Figure 1. Monitoring plots (100-m radius), water features, and upland ecological sites at Browns Park National Wildlife Refuge, 97 km northwest of Maybell, Colorado. Loamy (n 5 26), Saline Lowland (n 5 11), Sandy (n 5 31), and Sandy-Skeletal (n 5 39) were delineated by soil texture characteristics and plant species composition.

64(2) March 2011 111 Songbird Surveys potentially plausible predictors of songbird species density and Songbirds were surveyed in each of the 107 plots during the diversity based on their typical habitat associations. Mallow’s breeding season using fixed-radius (100-m) point-sampling C-p (a measure of model fit), standard deviations, and adjusted methods (Bibby et al. 2000; Buckland et al. 2001). Survey dates R2 were used to identify the best model with up to four site were 6–11 and 26–31 May 2006 and 6–11 and 23–27 May characteristics in best subset regression methods. Easily 2007. One observer located at the plot center (same individual) interpreted models with the least number of characteristics surveyed songbirds between 0600 and 1000 hours on days with were selected and fit with residual analysis and variance no precipitation and wind speeds less than 20 km ? h21. inflation factor (VIF) values to assess multicollinearity Following a 1–2-min settling period, locations of all singing (VIF . 5.0). Where density of any songbird species or songbird male songbirds were identified within 100-m radius from the diversity differed among ecological sites, the regression models center of each plot for 5 min. Distances from the plot center were run using a GLM with the songbird metric as the response were estimated using a laser range finder. variable, site characteristics as covariates, and ecological site as a categorical fixed factor to determine if ecological site adds additional power beyond what the site characteristics provide. Data Analysis We tested for differences in 60 site characteristics (response variables) among ecological sites (fixed factor) using general RESULTS linear models (GLM) to account for unequal number of plots in each site. The number of plots within ecological sites was Site Characteristics approximately proportional to the area of each site on the refuge. Twenty-nine of the 60 measured site characteristics differed We tested for differences between observed years; results among ecological sites (Table 1). Although we anticipated that supported pooling of site characteristics data across year. ecological sites would differ in site characteristics, we wanted Residuals were assessed for meeting the assumptions of GLM. to determine which characteristics (and associated methods) When models detected differences (alpha # 0.05), ecological site consistently discerned among ecological sites and how these means were separated using Fisher’s Protected LSD test. characteristics related to songbirds. Wyoming big sagebrush Songbird densities were derived using standard distance (Artemisia tridentata ssp. wyomingensis [Beetle and Young] estimation methods for point transects (Buckland et al. 2001, Welsh), shrub cover, plant and shrub height, small canopy gaps 2009). We estimated density of the four most common species (, 100 cm), and shrub density differed among three sites for which sufficient observations (. 60) allowed estimating a (Table 1). Vegetation structure represented by gap (canopy and detection function: Brewer’s sparrow, vesper sparrow (Pooe- basal) and height together illustrate differences among four cetes gramineu), lark sparrow (Chondestes grammacus), and ecological sites in our study (Fig. 2). sage sparrow. Species densities were estimated with program DISTANCE 5.0 Release 2 (Thomas et al. 2005). Although we Songbird Density and Diversity assumed that the detection function for each species would not Twenty-nine songbird species and 1 070 male songbirds were vary between years, we modeled detection functions for each recorded over the 2 yr. We detected shrub-steppe- and year and evaluated their Akaike information criterion (AIC) grassland-associated species of concern (Paige and Ritter values as recommended by Buckland et al. (2001). Results were 1999; Colorado Division of Wildlife 2003; Boyle and Reeder in favor of pooling the data to obtain a detection function for 2005), including Brewer’s sparrow, green-tailed towhee (Pipilo each common bird species. Prior to modeling, at least 10% of chlorurus), horned lark (Eremophila alpestris), lark sparrow, the largest distances were truncated (Buckland et al. 2001). Key sage sparrow, sage thrasher (Oreoscoptes montanus), Say’s functions (half normal, hazard, and uniform) with hermite and phoebe (Sayornis saya), vesper sparrow, and western meadow- cosine adjustments were used to model detection functions for lark (Sturnella neglecta). Songbird diversity ranged from 0.75 2 each species. Model fit was evaluated with a x goodness-of-fit species ? ha21 on Saline Lowland sites to 1.18 species ? ha21 on test, and AIC with a second-order correction for small sample Sandy-Skeletal sites (Fig. 3). Diversity (F1,3 5 6.73, P , 0.001) size (AICc) was used to select the most robust model (Buckland was highest in Sandy-Skeletal, lowest in Saline Lowland, and et al. 2001). A uniform key function with two cosine medium in Loamy sites. Brewer’s sparrow was common (32% adjustment terms was modeled for Brewer’s sparrow density. of observations and an average density of 1.28 birds ? ha21), as Vesper sparrow densities were modeled with uniform key was vesper sparrow (18% and 0.45 birds ? ha21). Less common functions with one cosine adjustment term. Lark sparrow and songbirds were lark sparrow (8% and 0.21 birds ? ha21) and sage sparrow were modeled with half-normal, hermite polyno- sage sparrow (8% and 0.18 birds ? ha21). Of these species, only mial functions with no adjustment terms. Songbird diversity vesper sparrow densities did not differ among ecological sites 21 (species ? ha ) was calculated as the number of equally (Fig. 3). Brewer’s sparrow densities were higher in Saline common species required to generate the observed heterogene- Lowland and Loamy than in Sandy-Skeletal sites (F1,3 5 2.83, ity of the sample (Hill’s reciprocal index; Hill 1973; Krebs P 5 0.040). The species was detected in . 50% of the plots 1999). Density and diversity were used as response variables in across all ecological sites. Lark sparrow density (F1,3 5 2.78, GLM to determine differences by ecological site. P 5 0.040) was highest in Loamy and Sandy and lowest in Regression models were built with best subset regression Saline Lowland sites. Sage sparrow density (F1,3 5 11.58, methods using songbird diversity and density as response P , 0.001) was higher in Sandy-Skeletal sites than in Loamy variables and site characteristics as independent variables. and Sandy sites. They were not detected in Saline Lowland From the set of 60 site characteristics, we selected a subset of sites.

112 Rangeland Ecology & Management Table 1. Characteristics (6 SE of the mean) of four shrub-steppe ecological sites (no. of plots) in northwestern Colorado, measured in 2006 and 2007. Across ecological sites, characteristic means or medians followed by the same letter are not different (P $ 0.050). Characteristics in bold differ in at least three sites. Plant canopy height measured by line-point intercept; shrub canopy height measured by gap intercept. Plant and shrub canopy gaps are calculated as the percent of line covered in each gap size category; values will not total to 100%.

Characteristic P Loamy (26) Saline Lowland (11) Sandy (31) Sandy-Skeletal (39) Cover (%) Needle and thread , 0.001 3.7 (0.9) a 0.0 (0) b 0.2 (0.1) b 1.0 (0.5) b Squirreltail , 0.001 0.1 (0.1) b 0.0 (0) b 0.3 (0.1) b 1.1 (0.2) a All perennial grasses 0.033 4.2 (1.0) a 1.7 (1.3) ab 0.9 (0.2) b 3.0 (0.9) a Greasewood , 0.001 8.0 (2.2) b 39.1 (3.7) a 5.7 (1.1) b 4.3 (1.0) b Green rabbitbrush 0.002 0.1 (0.1) b 4.2 (2.1) a 0.2 (0.1) b 1.6 (0.6) b Shadscale , 0.001 1.1 (0.3) b 0.7 (0.5) b 5.2 (0.7) a 5.9 (0.7) a Spiny hopsage 0.001 3.8 (0.6) a 1.0 (1.0) b 3.8 (1.0) a 0.8 (0.3) b Wyoming big sagebrush , 0.001 4.1 (0.9) b 0.0 (0) c 0.6 (0.3) c 8.5 (1.3) a All shrubs , 0.001 17.3 (1.9) c 45.9 (3.0) a 16.0 (0.9) c 22.4 (1.3) b All perennial forbs and grasses 0.002 2.8 (0.7) a 2.8 (1.3) a 0.5 (0.1) b 1.4 (0.3) ab Total canopy , 0.001 32.8 (2.3) b 56.2 (4.0) a 33.9 (1.3) b 33.9 (1.6) b Bare ground , 0.001 42.0 (2.4) a 27.1 (3.0) b 39.8 (2.0) a 41.6 (1.8) a Litter 0.012 42.5 (2.8) b 52.1 (3.4) a 40.2 (1.9) b 39.5 (1.5) b Height (cm) Plant canopy 0.001 26.9 (1.9) b 40.5 (3.2) a 22.8 (0.9) c 20.2 (0.9) c Shrub canopy , 0.001 39.4 (2.4) b 63.2 (4.1) a 31.0 (1.4) c 28.9 (1.1) c Basal and canopy gaps (%) Plant canopy 20–50 cm 0.018 5.8 (0.8) b 5.1 (0.4) b 5.9 (0.6) b 7.8 (0.5) a Plant canopy 51–100 cm 0.001 11.9 (1.1) bc 8.7 (0.8) c 13.1 (0.9) ab 15.1 (0.7)a Plant canopy 101–200 cm , 0.001 18.4 (1.1) a 12.3 (0.9) b 20.0 (0.9) a 19.9 (0.9) a Plant canopy . 200 cm 0.001 26.1 (3.1) a 12.1 (2.5) b 23.0 (2.6) a 15.4 (1.2) b Shrub canopy 20–50 cm , 0.001 2.4 (0.3) c 4.4 (0.5) ab 3.1 (0.3) bc 5.8 (0.5) a Shrub canopy 51–100 cm , 0.001 6.9 (0.7) c 7.5 (0.8) c 9.2 (0.7) b 12.3 (0.8) a Shrub canopy 101–200 cm , 0.001 14.6 (1.1) b 11.6 (1.1) b 19.1 (1.0) a 19.6 (1.0) a Shrub canopy . 200 cm , 0.001 45.4 (2.7) a 18.6 (3.4) b 39.3 (2.0) a 24.2 (1.7) b Plant canopy median (cm) 0.001 98.1 (8.1) a 69.8 (4.9) b 88.5 (5.4) a 71.3 (2.2) b Shrub canopy median (cm) , 0.001 142.1 (10.1) a 82.1 (7.9) b 121.3 (4.8) a 92.9 (9.6) b Shrub basal median (cm) 0.003 502.0 (42.0) a 385.6 (35.3) b 550.1 (38.8) a 383.7 (31.6) b

Shrub density by height (no. ? ha21) 10–50 cm , 0.001 8 149 (1542) bc 3 988 (841) c 10 859 (978) b 19 228 (1856) a 51–100 cm , 0.001 3 320 (392) b 5 906 (601) a 2 037 (263) c 1 941 (229) c . 101 cm , 0.001 182 (58) b 1 433 (341) a 11 (5) b 26 (9) b

Songbirds and Site Characteristic Relationships characteristics. Brewer’s sparrow was abundant in open plant Vegetation structure represented as basal and canopy gaps, communities with scattered, tall shrubs (40–100 cm). Lark species cover, height, and shrub density were related to sparrows also favored open plant communities, particularly songbird density and diversity (Table 2). Sage sparrow and short shrub communities containing broom snakeweed. Short, vesper sparrow models included horizontal vegetation charac- dense herbaceous communities with scattered shrubs, including teristics, such as basal and canopy gap and plant species cover. rabbitbrush, were positively related to a diversity of songbird Sage sparrows were strongly associated with Wyoming big species. sagebrush and bottlebrush squirreltail (Elymus elymoides Regression models for Brewer’s and lark sparrow densities [Raf.] Swezey) communities with basal gaps . 200 cm. Vesper and songbird diversity were not improved with ecological site sparrows were positively tied to open-canopy shadscale as a factor (P . 0.050). Brewer’s sparrows, however, favored communities and negatively related to winterfat (Kraschenin- tall, open-canopy greasewood; Wyoming big sagebrush; and nikovia lanata [Pursh] A. Meense & Smit). Brewer’s sparrow, spiny hopsage communities, which were common in Saline lark sparrow, and songbird diversity models were closely tied Lowland, Loamy, and Sandy sites (Table 1). Lark sparrow to both horizontal (basal and canopy gap, plant cover, and densities and songbird diversity were supported by site shrub density) and vertical (height and shrub density) site characteristics common to Loamy, Sandy, and Sandy-Skeletal

64(2) March 2011 113 Figure 2. Generalized diagram of structural characteristics of four shrub-steppe ecological sites monitored with gap intercept and height measures. Axes represent average horizontal (basal and canopy gap on x-axis) and vertical characteristics (height on y-axis) of each site type. For example, Loamy sites support mixed height, open plant communities (basal gaps and shrub canopy gaps . 100 cm) with dense understory vegetation (plant canopy gaps , 100 cm). In comparison, Saline Lowland sites support tall (. 50 cm), closed-canopy (canopy gaps , 100 cm), and widely spaced plants (basal gaps . 100 cm) with very little understory vegetation. Some plant illustrations were taken from Stubbendieck and Hatch (1997), used with permission.

114 Rangeland Ecology & Management Table 2. Best subset regression models for songbird density (birds ? ha21) and diversity (no. species ? ha21)(n 5 107) surveyed in northwestern Colorado, 2006 and 2007. Median plant basal gap in meters. Plant canopy height (median) in decimeters measured with line-point intercept. Plant cover and gap in deciles. Variance inflation factor (VIF) values assess multicollinearity in models.

Response Model Coefficient SE1 P VIF s Adjusted R2 Brewer’s sparrow Constant 20.38 0.23 0.101 0.8 0.352 Plant basal gap (m) 0.15 0.06 0.012 1.2 Shrubs ? m22 . 100 cm tall 27.2 1.85 , 0.001 2.0 Plant canopy height (dm) 0.64 0.11 , 0.001 2.3 Vesper sparrow Constant 0.22 0.13 0.088 0.4 0.163 Shadscale (%) 0.28 0.12 0.018 1.4 Winterfat (%) 20.41 0.16 0.014 1.0 Shrub canopy gap 101–200 cm (%) 0.15 0.8 0.062 1.4 Lark sparrow Constant 0.12 0.06 0.069 0.3 0.151 Broom snakeweed (%) 2.2 0.61 , 0.001 1.0 Shrub canopy gap . 200 cm (%) 0.03 0.02 0.094 1.1 Shrubs ? m22 . 100 cm tall 20.79 0.46 0.09 1.1 Sage sparrow Constant 0.14 0.04 , 0.001 0.2 0.408 Wyoming big sagebrush (%) 0.17 0.03 , 0.001 1.1 Bottlebrush squirreltail (%) 0.79 0.19 , 0.001 1.1 Plant basal gap 101–200 cm (%) 20.07 0.02 0.007 1.1 Diversity Constant 1.52 0.07 , 0.001 0.3 0.273 Plant canopy height (dm) 20.17 0.03 , 0.001 1.2 Shrub basal gap 101–200 cm (%) 20.27 0.07 , 0.001 1.0 Green rabbitbrush (%) 0.25 0.08 0.003 1.2 1SE indicates standard error.

Figure 3. Songbird density and diversity (n 5 107; 6 1 SE of the mean) in Loamy (n 5 26), Saline Lowland (n 5 11), Sandy (n 5 31), and Sandy- Skeletal (n 5 39) ecological sites in northwestern Colorado. Across ecological sites, songbird diversity and density with the same letter are not different (P $ 0.05). Diversity is the reciprocal of Simpson’s index (Hill 1973; Krebs 1999).

64(2) March 2011 115 sites (short, open-canopy plant communities). Sage sparrow 1981; Walker 2004) are not supported by our study because densities increased with greater Wyoming big sagebrush cover, densities were high in ecological sites dominated by grease- and ecological site improved the regression model for sage wood and spiny hopsage. Its close association with basal gap, 2 sparrow density (adjusted R 5 0.460, F1,3 5 4.31, P 5 0.007). density of tall shrubs, and plant canopy height suggests it is selecting for these characteristics. Brewer’s sparrows prefer landscapes with ample cover of tall shrubs (50–150 cm tall) DISCUSSION (Paige and Ritter 1999; Chalfoun and Martin 2007), an observation consistent with our results. However, we observed Our results support the use of ecological sites as management low densities in plant communities with desirable tall shrubs units to characterize songbird habitat. Site characteristics that with undesirable horizontal characteristics (i.e., dense, closed- differ among ecological sites also differentiated habitat among canopy shrub communities). Lark sparrow habitat studies, the songbird species. Basal and canopy gaps and vegetation though limited, consistently report their dependence on both height were key structural characteristics that separated horizontal and vertical characteristics of shrub communities. ecological sites and distinguished songbird habitat even where Lark sparrow’s preference for open grasslands with scattered, there were similarities in plant composition among ecological tall shrubs (Paige and Ritter 1999; Martin and Parrish 2000; sites. Vegetation structure distinguished characteristics of both Righter et al. 2004; Beason et al. 2005) parallels our findings. ecological sites and songbird habitat (Fig. 2; Table 2). For Lark sparrows will abandon sites with complete shrub removal example, Loamy and Sandy-Skeletal sites had similar plant (Bock and Bock 1987) and sites with high shrub density species composition, but their horizontal (basal and canopy (Parrish et al. 2005). In a mixed-grass prairie, the majority of gap) and vertical (height) characteristics differed substantially. lark sparrow nests were associated with widely spaced broom Furthermore, we identified structural characteristics (horizon- snakeweed on grazed pastures (Lusk et al. 2003). In this study, tal, vertical, or both) important to songbirds even for songbird lark sparrows favored broom snakeweed cover but only in species that adhered to specific plant species associations. For short (, 100 cm), open-canopy plant communities. Very few example, sage sparrow favored sagebrush-dominated ecologi- lark sparrows were documented in Saline Lowland sites, as they cal sites but was associated with plant basal gap. Sage sparrows support tall, dense shrub communities. are thought to depend on large, continuous stands of tall Songbird diversity was closely associated with the structure sagebrush with little grass cover (Rich 1978; Knick and of shrub communities rather than to specific shrub species—a Rotenberry 1995; Paige and Ritter 1999) as opposed to mixed common finding in shrub-steppe songbird research (Walcheck shrub or salt-desert communities (Wiens and Rotenberry 1981; 1970; Best 1972; Parrish et al. 2005). We noted greater Vander Haegen et al. 2000). Based on the literature, we songbird diversity in structurally diverse shrub communities expected higher sage sparrow densities in closed-canopy than in uniform, monotypic shrub communities, consistent sagebrush communities. Instead, sage sparrows responded with the literature (Wiens and Rotenberry 1981). Shrub-steppe positively to closed-canopy and widely spaced mixed shrub songbirds require structural elements of habitat, and where communities. Vesper sparrows were common across the study there is complete removal of shrubs (either live or dead), area, but we found that horizontal structure (shrub canopy gap) Brewer’s sparrows, lark sparrows, and sage sparrows abandon was just as important to vesper sparrow as was plant the area (Walcheck 1970; Bock and Bock 1987; Martin and composition. Vesper sparrows, considered a grassland songbird Parrish 2000). Songbird diversity has been linked to horizontal (Vander Haegen et al. 2000; Beason et al. 2005), are found in a and vertical plant community characteristics (MacArthur and variety of plant communities (Paige and Ritter 1999), but there MacArthur 1961; Roth 1976), and our results extend this link are few reports of their association with vegetation structure. to the context of ecological sites. Because state-and-transition Though vesper sparrows are common in shrub steppe, many models are not currently developed for the ecological sites at studies have found that changes in shrub composition and Browns Park, we could not test specific relationships between density (e.g., shrub cover reduction or removal by chemical or songbirds and state-and-transition models. We recognize that fire treatments) have little effect on its abundance (Best 1972; alternate states will alter site structural characteristics and Earnst et al. 2009). Our results at Browns Park suggest that wildlife habitat on any ecological site. Structural characteristics sage sparrows and vesper sparrows were selecting not for or will likely shift dramatically in alternate states within an against mixed-shrub communities but more for their horizontal ecological site and, so too should songbird density and structural characteristics. diversity. Once state-and-transition models are fully developed Vegetation structure greatly influenced Brewer’s sparrow and and tested, they will provide more information about the lark sparrow, and both preferred open-canopy plant commu- variety of potential habitat within a given ecological site. nities with tall, scattered shrubs. Throughout this study, we Managers can adjust songbird habitat by managing for specific discovered structural characteristics that add to our under- states or a variety of states on ecological sites. For example, at standing of species-specific songbird habitat. Brewer’s sparrow Browns Park, management efforts to maintain or increase the and lark sparrow responded more strongly to horizontal and extent of shrub community structural diversity should accom- vertical characteristics of the plant communities than to species modate several songbird species. composition. Brewer’s sparrow is considered a sagebrush Our study diverges from earlier shrub-steppe songbird obligate but occurs in mixed-shrub communities (Wiens and research primarily because we quantified structural character- Rotenberry 1981; Medin 1990; Parrish et al. 2005). Earlier istics by combining basal and canopy gap with height observations that Brewer’s sparrows are negatively related to measurements, allowing us to discern between associations of greasewood and spiny hopsage cover (Wiens and Rotenberry songbirds with plant species composition versus vegetation

116 Rangeland Ecology & Management structure. The array of ecological sites at Browns Park offers a LITERATURE CITED variety of shrub communities to the songbirds and perhaps a greater variety than has been previously reported in any single BEASON, J. R., R. LEVAD, AND T. LEUKERING. 2005. Monitoring Colorado’s birds: the study (e.g., Rotenberry and Wiens 1980; Wiens et al. 1986; 2004 field season report. Brighton, CO, USA: Rocky Mountain Bird Vander Haegen et al. 2000). The vegetation structure we Observatory. 66 p. documented clearly discerned shrub-steppe songbird habitat. BEST, L. B. 1972. First-year effects of sagebrush control on two sparrows. Journal of Wildlife Management 36:534–544. Visual obstruction and line-point intercept commonly used to BESTELMEYER, B. T., J. R. BROWN,K.M.HAVSTAD,R.ALEXANDER,G.CHAVEZ, AND record structure in other studies can fail to capture the horizontal J. E. HERRICK. 2003. 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